The present invention relates to an AC power supply for lighting a cold-cathode tube and, in particular, to a cold-cathode tube lighting circuit having an inverter using a piezoelectric transformer as an inverter transformer.
As is well known in the prior art, an inverter comprises a transformer and a switching circuit for switching a DC input for driving the transformer at a controlled switching frequency. Thus, a DC/AC inverted power is taken out from the transformer. The transformer is called an inverter transformer.
A cold-cathode tube is used as a backlight for a liquid crystal display (LCD) used in a personal computer, a word processor or other electronic devices, especially, of a notebook type.
In order to meet the demand for small-sized and light-weight devices, a piezoelectric transformer has become used as the inverter transformer in the cold-cathode tube lighting circuit.
However, there has been a problem due to characteristics of the cold-cathode tube that the cold-cathode tube is difficult to light at a start when the inverter is powered on. This problem is notable at a relatively low ambient temperature where the current hardly flows through the cold-cathode tube. When the cold-cathode tube does not light, the piezoelectric transformer is kept open at its output so that the piezoelectric transformer is damaged in the worst case.
On the other hand, the known cold-cathode tube lighting circuit often has a light control circuit or a dimmer circuit. The dimmer circuit controls the switching operation in the inverter so that the switching operation is intermittently stopped at a dimmer frequency. In detail, the dimmer circuit generates a pulse signal as a dimmer signal having the dimmer frequency of a relatively high but sufficiently lower than the switching frequency. A duty ratio of the dimmer pulse signal is controlled to a desired value selected by a manual selector. Thus, the switching operation is performed and stopped every ON duration and every OFF duration, respectively, of the dimmer pulse signal. The piezoelectric transformer intermittently supplies its AC output power to the cold-cathode tube. The cold-cathode tube repeatedly flushes every ON duration at the dimmer frequency. Therefore, it is possible to adjust the brightness of the cold-cathode tube by selecting a desired duty ratio by the manual selector.
In the liquid crystal display, displaying is made through scanning using a driving signal. If a frequency of the scanning in the liquid crystal display and the dimmer frequency do not have a constant relationship, interference fringes appear on a screen of the liquid crystal display by light interference caused due to a difference between both frequencies.
For example in a monitor of a liquid crystal display, the scanning frequency is typically 1 kHz to 100 kHz while the dimmer frequency is 100 Hz to 10 kHz. However, there has been inconvenience that a higher-order frequency component of the dimmer signal is nearly equal to but slightly different from the scanning frequency to cause the interference fringes on the liquid crystal display.
The problem could be avoided by changing the dimmer frequency in the dimmer circuit depending on the scanning frequency of the liquid crystal display.
However, since there are a number of types of the liquid crystal display having various scanning frequencies, it is difficult to adjust the dimmer frequency in the dimmer circuit in the cold-cathode tube lighting circuit for any type of the liquid crystal display, resulting in increase of the cost.
Another known approach for preventing appearance of the interference fringes is to insert a transparent conductive sheet such as ITO (In.sub.2 O.sub.3 :Sn) film between a panel of the liquid crystal and the cold-cathode tube.
However, the transparent conductive sheet need to increase in size according to large size of the liquid crystal panel. This also results in increase of the cost.